Twisted-pair copper cables were initially employed to carry low-bandwidth voice signals. Today, they are widely used to carry high-speed broadband data signals from a central office (CO) or remote terminal (RT) to customer premise equipment (CPE) and vice versa in digital subscriber line (DSL) systems. The high-speed data communication over copper uses wider bandwidths, up to a few megahertz (MHz) in asymmetric digital subscriber line (ADSL), a few tens of MHz in a very-high-bit-rate digital subscriber line (VDSL/VDSL2), and a 100 MHz or greater in G.fast, a standard currently in development and in which many aspects are finalized. At higher frequencies, crosstalk coupling between adjacent pairs increases which may significantly degrade both data rate and stability performance. International Telecommunication Union (ITU) standard G.993.5 defines procedures and protocols, so called vectoring, to allow far-end crosstalk cancellation among VDSL2 modems, which may enhance performance.
The G.fast standard adopted time division duplexing (TDD) to transmit data signals in downstream and upstream directions. As such, the signal transmission in the time-domain is discontinuous and the entire available bandwidth may be used in both directions. The combination of TDD duplexing with the very high bandwidth and low transmit power of G.fast provides the ability to employ discontinuous mode power savings when using G.fast. For example, in discontinuous operation, not all of the time available for data transmission is utilized, for example, when user traffic is lower than the channel capacity, the entire system transmission and/or reception path may be turned off during some of the digital multi-tone (DMT) symbols within a TDD frame to save power in a digital front-end (DFE), an analog front-end (AFE), and/or a line driver (LD).
A frequency division duplex (FDD) system in comparison divides the communication channel in a frequency domain between each direction and transmission in time may be continuous. However, for power saving or other purposes, an FDD system may operate in discontinuous transmission, and VDSL2 systems are example FDD systems that may employ discontinuous transmission.
In low-power link states (LPLSs) in systems that employ discontinuous transmission, there may be no activity (e.g., no data signal communications) for some period of time between two modems.